As a next-generation power generation system, there is known a fuel cell module that generates electricity using hydrogen-containing gas and oxygen-containing gas. This fuel cell module houses a fuel cell in a container thereof. In addition, as the temperature in the fuel cell is raised, cell voltage per current density increases. Due to this, power generation efficiency of the fuel cell becomes high. On the other hand, if the temperature of the fuel cell becomes too high, durability becomes low. For this reason, it is necessary to maintain the fuel cell within an appropriate temperature range.
In addition, hydrogen-containing gas to be supplied to the fuel cell is generated in a reformer. The reformer uses heat of exhaust gas generated by combustion of the gas that has not been consumed in the fuel cell.
In other words, in the reformer, hydrocarbon such as methane (CH4), which is a raw fuel, and steam are caused to react by the steam reforming process using heat of exhaust gas. This reaction generates hydrogen and carbon monoxide (CO). On the other hand, as the combustion heat quantity of exhaust gas becomes low, the temperature of the reformer becomes low. Due to this, the quantity of methane or the like that is not reformed by the reformer increases. The unreformed methane causes a reforming reaction that is associated with heat absorption in the fuel cell and reduces power generation efficiency.
Furthermore, in the steam reforming process, carbon is deposited when the amount of steam becomes less than double the amount of carbon of methane or the like by mole ratio. This carbon deposition is referred to as carbon coking. If carbon coking is generated, the amount of hydrogen and carbon monoxide to be supplied to the fuel cell is reduced, and thus power generation efficiency may be reduced and the fuel cell may be degraded. For this reason, it is necessary to supply in a stable manner the amount of steam that is equal to or greater than double the amount of fuel carbon by mole ratio.
The problem to be solved by the present invention is to provide a fuel cell system that is capable of improving power generation efficiency.